Ok, I don't know how many people this is going to help, because my application seems to be a lot simpler than most of the ones described here, but this is what I've experienced.
I downloaded iTunes for the very first time onto my Windows machine the other day. All I've downloaded via iTunes are individual songs. The very first one got stuck in processing. I ignored it a) because I could play the song, and b) I could continue to download other songs. I downloaded several more, and another got stuck in processing.
I wanted to see what I could do to resolve it, so I closed iTunes and re-booted. Immediately afterwards, I decided to test things by making a playlist out of the downloads, and burning a CD. When I went to add the songs that had been hung in processing, I got a msg about it needing to download them from the Cloud (although thay were obviously listed with the other songs) - ok, fine. Everything worked perfectly, and when I went to the iTunes store, the songs I had downloaded all said "Play", whereas, before the ones that were stuck in processing had said "Downloading".
Ok, so I downloaded a few more, with the second getting stuck in processing. So, without closing iTunes, I tried to make a playlist, and burn a CD. Again, when I went to add the stuck song, it offered to load it from the Cloud, which it did. But when I tried to burn a CD, it said it could only burn songs that were on my hardrive to the CD. (?) So, I closed iTunes, and killed the task via Taskmanager so I didn't have to reboot. I re-started iTunes, and deleted and re-made the 2nd playlist, and the CD burned without a problem. Again, the song that was hung in processing displayed the "Play" button (as it should have), rather than the "Downloading".
And, yes, all 3 songs that got stuck in processing were, and *still are*, listed in the iTunes Download folder, but at this point, I don't see any consequence to that, or any evidence that any of the songs ever got stuck in processing.
Hope this helps someone, and maybe you guys with bigger issues can use the info to narrow down your search for answers.